1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell. More particularly, in the polymer electrolyte fuel cell, an oxygen-containing hydrocarbon is introduced as a material for fuel from a supply section for supplying the material for fuel. The material for fuel is decomposed by a biochemical catalyst to generate hydrogen as fuel before the material for fuel reaches an anode of the polymer electrolyte fuel cell, and the generated hydrogen is supplied to the anode.
2. Description of Related Art
A fuel cell is provided with a cathode and an anode on both sides of an electrolyte. The cathode (oxidizer electrode) is supplied with an oxidizing gas such as oxygen, air or the like and the anode (fuel electrode) is supplied with a fuel such as hydrogen, a hydrocarbon or the like, so that an electrochemical reaction is induced to generate electricity and water.
Fuel cells are classified into a number of groups such as alkaline fuel cells, acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and polymer electrolyte fuel cells (PEFCs) according to their types of electrolytes. Of these fuel cells, the PEFCs have proton-conductive solid polymers as electrolytes and are systems using high-purity hydrogen gas as fuel.
Since the PEFCs can work effectively at low temperatures and have high output density, the PEFCs are very likely to be put in practical use for power generation for vehicles and for small-scale houses. However, the PEFCs have the disadvantage of requiring huge cylinders containing compressed hydrogen for supplying gaseous hydrogen as fuel. Or alternatively, hydrogen may be liquefied and stored in cylinders. However, the liquefaction of hydrogen needs cooling at an extremely low temperature of −253° C. Furthermore, because liquefied hydrogen evaporates easily and leaks from gaps between metal molecules of the cylinders, hydrogen is consumed significantly. In an alternative way, hydrogen may be stored in a special metal referred to as a “hydrogen-occulusion alloy.” However, in order to store a sufficient amount of hydrogen, a large amount of the alloy is required, and consequently, fuel supply systems become heavy (by Junji Kayukawa, Trigger, July 2000, page 14, THE NIKKAN KOGYO SHINBUN, LTD.). For the above-mentioned reasons, the PEFCs have some problems with their fuel supply systems, and at present it is difficult to put the PEFCs into widespread use as transportable power sources.
There are reforming processes of using liquid fuels containing hydrogen and decomposing the liquid fuels to generate hydrogen. The reforming processes include a steam reforming process of applying steam of extremely high temperature for inducing reaction and a partial oxidizing process of feeding oxygen for inducing reaction. Methanol can be reformed at a relatively low temperature of 300° C. as compared with gasoline, gas oil, propane, butane and methane. Since the temperature is still high, the size of reforming devices is difficult to reduce.
On the other hand, direct methanol-air fuel cells (DMFCs) are directly supplied with methanol as fuel. Since they can use proton-conductive polymers as electrolytes, the DMFCs can possibly work at temperatures lower than 100° C. Since the fuel is liquid and is easy to transport and store, the DMFCs are considered to be suitable for size reduction and transportabilization. Thus the DMFCs are regarded as very likely power sources for automobiles and power sources for mobile electronic equipment.
Direct methanol-air fuel cells using proton-conductive polymer membranes as electrolytes (PEM-DMFCs) have a structure in which porous electrodes carrying electrocatalysts are formed on both faces of a membrane of a fluorinated polymer having sulfonic acid groups, for example, a thin membrane such as Nafion® manufactured by DuPont, in such a manner as the porous electrodes sandwich the polymer membrane, the anode is directly supplied with an aqueous methanol solution and the cathode is supplied with oxygen or air. At the anode, methanol reacts with water to generate carbon dioxide, protons and electrons:CH3OH+H2O→CO2+6H++6e−.At the cathode, oxygen reacts with protons and electrons to generate water:3/2O2+6H++6e−→3H2O.These reactions progress with the help of the electrocatalysts carried by the electrodes. The theoretical voltage of these reactions is 1.18 V, however in practical cells, the actual voltage is lower than the theoretical voltage for various reasons.
Platinum catalyzes the reaction of methanol with water and is an excellent anode catalyst. General mechanism of the reaction of platinum with methanol is represented by the following chemical formulae:Pt+CH3OH→Pt—CH2OH+H++e−Pt—CH2OH→Pt—CHOH+H++e−Pt—CHOH→Pt—CHO+H++e−Pt—COH→Pt—CO+H++e−Pt—CO+H2O→Pt+CO2+2H++2e−
However, the surface of the platinum electrocatalyst is poisoned with CO generated from methanol during the reactions. Consequently, the reaction area of the platinum electrocatalyst decreases, and therefore, the performance of cells declines.
In order to prevent the platinum electrocatalyst from being poisoned with CO, measures are taken to improve a surface structure of the platinum electrocatalyst or to add different metals such as Ru, Sn, W and the like to platinum. However, the different metals have lower catalytic activities to methanol than platinum, and to compensate that, the reaction temperature needs to be elevated. If the reaction temperature is high, methanol penetrates through the proton-conductive polymer electrolyte membrane (Nafion® membrane, Dow® membrane, Aciplex® membrane, Flemion® membrane) from an anode side of the membrane to reach the cathode, and directly reacts with an oxidizer on the electrocatalyst of the cathode. This phenomenon is referred to as cross-over, which is a short-circuit problem. Also, the elevation of the reaction temperature is not suitable for power sources for mobile electronic equipment which needs to be operated at relatively low temperatures.
Bacteria such as Clostridia and Bacilli are known to decompose oxygen-containing hydrocarbons and produce water and carbon dioxide through sugar fermentation (Nikkei Latest Biotechnological Terms Dictionary, 4th edition, edited by Nikkei Bio-tech, Nikkei Business Publications, Inc., page 346). In order to measure the amount of hydrogen produced by such bacteria, an example is reported in which the produced hydrogen is supplied to the anode of a fuel cell and the amount of generated electricity is measured. However, this is not put into practical use as a polymer electrolyte fuel cell (Japanese Unexamined Patent Publication No. HEI 7(1995)-218469).
Accordingly, there is a demand for a polymer electrolyte fuel cell which uses an oxygen-containing hydrocarbon such as methanol as a material for fuel and can generate electricity at low temperatures with good efficiency.